Farah and Marshall are set to return from back and knee injuries respectively for the match against the Knights at Energy Australia Stadium.
"They'll have their own demons to face during the week, they obviously haven't played a lot of footy so it's not an automatic that they'll be back at their best," Sheens said.
"It's nice to have the boys back but they've got a lot of work to do on their game and everyone else has to lift their game to carry them through.
Marshall has spent the majority of the season on the sidelines after sustaining a knee injury in round one which kept him out for five weeks.
And the five-eighth re-injured his knee in his comeback game against Cronulla in round seven.
"I'm feeling confident this week," Marshall said after being named in the starting side.
"We've been put through a pretty extreme training and we've done the hard yards and now it's about getting on the field and doing it out there."
Farah has been sidelined for over a month with a back injury.
"I've been training with the team for a couple of weeks now," Farah said.
"I saw a specialist last week and he's cleared me to come back and play so now it's just a matter of getting on the field and getting some fitness under my belt."
Marshall warns he and Farah lack match fitness and may take some time to hit top form.
"A lot of people are going to be expecting miracles from us but our main goal is just to get through the game.
"We don't want to put too much pressure on ourselves, just because we're back doesn't mean that we're going to set the world on fire."
"Newcastle is a great side, and at home, even harder to beat."